1. Field of the Invention
The present invention relates to a mechanical devices for replacing the natural functions of kidneys, and in particular, is related to a portble artificial kidney designed for continuous use and worn on the body of the patient.
2. Description of the Prior Art
Different types of artifical kidneys are presently known in the art. Generally, they contain a material exchanger which is placed somewhere in the circulating blood stream and include a semi-permeable membrane. An external liquid system functions to collect the materials passed out from the blood and replaces the quantity of liquid which has been removed. Large amounts of fluid must be separated out of the blood, and until now, all the devices in use are stationary devices which are rather large and require the patient to be connected thereto, for example, three times per week for several hours in each case. Although there has been a great need for a portable material exchanger to function as an artifical kidney, none have been available until the present time, which may be worn by the patient and used continually.
The method predominately used in the prior art was based upon the principle of dialysis. With dialysis, the patient's blood is passed along on one side of a semi-permeable membrane and a dialyzing solution of suitable composition is passed along on the other side. The material exchange, more particularly the transition (transfer) of uriniferous substances out of the blood through the semi-permeable membrane into the dialyzing solution, is thus predominately a process of diffusion.
In recent years another method has proved to be suitable to accomplish the same purpose. This method utilizes convective transportation of material, is called diafiltration or hemofiltration and provides for the drawing off from the patient of fairly large quantities of plasma water with uriniferous substances dissolved therein. The plasma water is drawn off through a filter membrane having a pressure differential thereacross. Simultaneously with the drawing off of the plasma water a replacement fluid is infused with a suitable replacement solution until the patient's fluid balance is normalized (brought back to normal). This method is more effective than the dialysis process when suitable filters are utilized, because less quantities of fluid need be withdrawn in order to obtain the same effective amount of filtering. With dialysis treatment approximately 160 liters of dialyzing solution are required, whereas to obtain the equivalent hemofiltration approximately 15 liters of substution solution are sufficient. In the future, it is expected that by using ketoamino acids together with hemofiltration this quantity of fluid may be reduced still further, since the nitrogen contained in the lower molecular weight uriniferous substance may be partially returned to the patient. As a result, the concentrations of lower molecular weight poisons occurring in the urine, which were previously a factor in obtaining further reduction of the fluid exchange, can be lowered initially.
With the type of hemofiltration previously used the treatment takes place discontinuously in a manner similar to hemodialysis and, in fact, generally takes place for periods of five hours, three times per week. Typically, approximately 17 liters of filtrate are drawn off per treatment and approximately 15 liters of substitution solution are supplied. Extending the duration of a treatment beyond approximately five hours is not practicable. As a result of the dilution effect of the substitute solution the effectiveness drops considerably in the course of treatment. The concentration of uriniferous substances in the filtrate is reduced to approximately one-third if this time period is extended. The average or mean value of the concentrate in the filtrate is approximately equal to 60% of that occurring at the start. Because of the limited difusion speed, the washing of the uriniferous substance out of the intercellular area proceeds slowly.
With a continuous treatment method, according to the principles of the present invention, the dilution effect is insignificant so that the same effective treatment is achieved with a small fluid exchange. The discontinuous application of the prior art method requires approximately three times seventeen liters of filtrate or about 50 liters in total. With the continuous filtration method, the quantity of filtrate required is only approximately 30 liters to remove the same quantity of uriniferous substances from the blood stream. This corresponds to a filtrate flow of approximately 3 milliliters per minute which can be achieved with a relatively small filter surface area.
The prior art techniques which require providing a connection between the circulation of the patient's blood and the filter, which is normally placed outside the body, represents a substantial problem. Generally, two hollow hypodermic needles are used; one for supplying the blood and one for returning the blood into the body. In order to obtain a sufficiently low flow through resistance, the diameter of these hypodermic needles are approximately 1.8 to 2.4 milimeters. Repeated puncturing with these large hollow needles is very painful for the patient and involves permanent damage to the tissue and the blood vessels punctured. This fact, in addition to the preceding quantitative observations with respect to the utilization of continuous filtration techniques, led to the development of a filter catheter which can be placed inside the blood circulation system of the patient for relatively large periods of time.